During normal breathing, mucous membranes which line the upper airway heat and humidify the inspired air so that it is at or near body temperature (37 degrees C.) with about 100% relative humidity when it enters the lungs. However, when a patient is undergoing mechanical ventilation, the inspiratory gases may be delivered through an invasive artificial airway device (such as an endotracheal tube or tracheostomy tube) such that they bypass much of the mucosal tissue of the upper airway, or sometimes through non-invasive airway devices, such as those used to deliver Continuous Positive Airway Pressure (CPAP) and high flow therapies. Because inspired gases delivered from mechanical ventilators or flow generators can be relatively cold and dry, it is typically desirable to heat and humidify the inspired gases before they enter the lungs. Insufficient humidification of the inspired gases can lead to clinically significant loss of body heat and water, with resultant inspissation of airway secretions, hypothermia and impaired pulmonary gas exchange due to alveolar atelectasis. In patients receiving respiratory therapy through mechanical ventilation or flow generators, heating and humidification of the inspired gases is usually accomplished by either a heat and moisture exchanger (HME) or a heater/humidifier (HH). With either of these systems, it is generally desirable to ensure that the inspired gas contains at least about 30 mg of water for each liter of inspired gas delivered at 33+2° C. Generally, HME's are used only for short periods of time (less than 24 hours), such as during transport of an intubated patient or during postoperative anesthesia recovery.
HH's are typically used when longer term mechanical ventilation is required or when use of an HME is contraindicated. HME's are passive humidification systems wherein a hygroscopic filter (e.g., hygroscopic foam or paper material that may be treated with salts) positioned at the out end of the artificial airway retains heat and moisture as the patient exhales. On the next inspiratory cycle, the dry and cold inspired gases from the ventilator are heated and humidified by the heat and moisture retained in the HME from previously exhaled breaths. HH's are active humidification and heating systems in which heat and vapor generating apparatus are used to add controlled amounts of heat and humidity to inspired gases as they are delivered to the patient. Cold and dry inspired gases from the ventilator pass through a humidification apparatus where water vapor (and some heat) is added to the inspired gases. In some cases, a heated wire is placed in the inspiratory conduit to maintain the temperature of the inspired gases and to minimize water condensation as the inspired gases are delivered to the patient airway.
An important aspect of any modern HH system is the precision with which a user may control the operation of the humidifying and heating apparatus and monitor the temperature and humidity of inspired gases at critical locations in the respiratory circuit. One commercially available HH system is the MR850 Respiratory Humidifier (Fisher & Paykel Healthcare, Auckland, New Zealand). This system compromises a humidifier that has a heater and a water chamber and a control panel, a heating wire positioned in the inspiratory conduit of the ventilation circuit and temperature sensors for monitoring temperature at the humidifier and at the patient airway end of the inspiratory conduit. The control panel includes a digital temperature display capable of displaying only one temperature at a time and several control buttons (e.g., a power on/off button, mode button and mute button) which the user must know how to manipulate in specific ways in order to bring about specific desired outcomes.
Other examples of HH systems and/or components thereof are described in U.S. Pat. No. 4,621,632 (Bartels, et al.) entitled Humidifier System; U.S. Pat. No. 5,857,062 (Bergamaschi, et al.) entitled Heated Respiratory Therapy Humidifier, U.S. Pat. No. 7,106,955 (Thudor, et al.) entitled Humidity Controller; U.S. Pat. No. 7,428,902 (Du, et al.) entitled Humidifier System for Artificial Respiration; U.S. Pat. No. 7,962,018 (Hunt, et al.) entitled Humidity Controller; U.S. Pat. No. 8,616,202 (Tatkov, et al.) entitled Control of Humidifier Chamber Temperature for Accurate Humidity Control, the entire disclosure of each such patent being expressly incorporated herein by reference.
There exists a need in the art for the development of new HH systems having enhanced control and monitoring capabilities for improved patient ventilation, safety and ease of use.